1. Field of the Invention
The present invention relates to a magnetic garnet single crystal and a Faraday rotator utilizing a magneto-optical effect which uses the magnetic garnet single crystal. The Faraday rotator using the magnetic garnet single crystal is used in a magneto-optical element such as, for example, an optical isolator, an optical circulator, an optical attenuator or the like.
2. Description of the Related Art
In an optical communication or an optical application device using semiconductor laser, an optical isolator, an optical circulator or an optical attenuator is widely used. The Faraday rotator can be cited as one of the essential elements for these devices.
Although a YIG (yttrium iron garnet) single crystal and a bismuth (Bi)-substituted rare earth iron garnet single crystal are known for the Faraday rotator, at present, the Faraday rotator using a bismuth-substituted rare earth iron garnet single crystal film formed by a liquid-phase epitaxial (LPE) method is the mainstream.
For example, in the Japanese Patent Publication No. 6-46604, a bismuth-substituted rare earth iron garnet is described. The bismuth-substituted rare earth iron garnet which is grown by a liquid-phase epitaxial growth method has a composition expressed by a general formula R3xe2x88x92(a+b)PbaBibFe5xe2x88x92cMcO12xe2x88x92d (R is at least one kind of element selected from rare earth elements and elements substitutable with the rare earth elements, M is at least one kind of element selected from elements substitutable with iron elements, a is a number between 0.01 and 0.2, b is a number between 0.5 and 2.0, c is a number between 0.01 and 2.0 and d is a number between 0 and 1). At the same time, a part of M in the aforementioned formula contains a tetravalent element which is belonging to the Group IVA and the Group IVB in the periodic table excluding Pb in which tetravalent element is more than 0.01 as an atomic ratio (c) in the above general chemical formula.
As disclosed in the above publication, Pb4+ can be eliminated when the Bi-substituted rare earth iron garnet single crystal is grown by the liquid-phase epitaxial method by adding an IV Group element. Therefore, an absorption loss at the time of transmitting light through the Bi-substituted rare earth iron garnet single crystal can be reduced.
By the way, as shown in the example disclosed in the Japanese Patent Publication No. 6-46604, if for example TiO2 is added as the IV Group element and a single crystal epitaxial film is grown by the liquid-phase epitaxial method, an effect of reducing the light absorption loss of the obtained Bi-substituted rare earth iron garnet single crystal epitaxial film is recognized. However, when the film thickness of the obtained epitaxial film is more than approximately 200 xcexcm, many crystal defects are confirmed on a film surface. When the Faraday rotator for an optical isolator is fabricated by polishing the surface of such a crystal and forming a non-reflective film, many defects are confirmed in the interior of the Faraday rotator according to an observation using the infrared ray and a reduction of the extinction ratio is also identified.
The invention described in the Japanese Patent Publication No. 6-46604 cites a reduction of the light absorption loss of the Bi-substituted rare earth iron garnet single crystal epitaxial film as a technical subject and none is disclosed with respect to the subject to suppress the generation of crystal defects and to improve the extinction ratio. If the generation of crystal defects in the Bi-substituted rare earth iron garnet single crystal epitaxial film can be suppressed, the extinction ratio of the Faraday rotator can be improved and furthermore, the performance of optical communication parts including the optical isolator can be improved by improving the extinction ratio of the Faraday rotator.
An object of the present invention is to provide a magnetic garnet single crystal which reduces an absorption loss by adding a tetravalent element and then suppresses a generation of crystal defects.
Another object of the present invention is to provide a Faraday rotator which reduces an insertion loss and improves an extinction ratio.
Therefore, the inventors of the present invention investigate to obtain a single crystal more than approximately 200 xcexcm which is required to fabricate the Faraday rotator without generating many crystal defects as well as additives for achieving a reduction in light absorption.
As a result, it is found that a remarkable effect is achieved if Pt which can steadily take a tetravalent structure similar to the structure of the IV Group elements is used as an additional element. In other words, when PtO2 or Pt is melted in the flux and a Bi-substituted rare earth iron garnet single crystal which is more than 200 xcexcm in film thickness is grown, the number of crystal defects on the epitaxial film surface is considerably reduced, no crystal defect is confirmed even if the single crystal is observed by the polarization microscope using the infrared ray, and the light absorption loss can be substantially naught (zero).
Further, it is found that if Ge is used as an additional element, a remarkable effect is achieved. In other words, when GeO2 is added and the Bi-substituted rare earth iron garnet single crystal which is more than 200 xcexcm in film thickness is grown, the number of crystal defects on the epitaxial film surface is considerably reduced, no crystal defect is confirmed either according to the observation of the interior of the single crystal by the polarization microscope using the infrared ray, and the light absorption loss can be substantially naught (zero).
The above objects are achieved by a magnetic garnet single crystal which is grown by the liquid-phase epitaxial growth method and expressed by a general formula BiaPbbA3xe2x88x92axe2x88x92bFe5xe2x88x92cxe2x88x92dBcPtdO12 (A in the formula is at least one kind of element selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, B is at least one kind of element selected from Ga, Al, Sc, Ge and Si and a, b, c and d are 0.8 less than a less than 1.4, 0 less than bxe2x89xa62.0, 0xe2x89xa6cxe2x89xa60.9 and 0 less than dxe2x89xa62.0 respectively).
In the above magnetic garnet single crystal according to the present invention, the film thickness is more than 200 xcexcm. Further, in the above magnetic garnet single crystal according to the present invention, 0.5xe2x89xa6b/dxe2x89xa62.0 is realized.
Furthermore, the above objects are achieved by a magnetic garnet single crystal which is grown by the liquid-phase epitaxial growth method and expressed by the general formula BiaPbbA3xe2x88x92axe2x88x92bFe5xe2x88x92cxe2x88x92dBcGedO12 (A in the formula is at least one kind of element selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, B is at least one kind of element selected from Ga, Al, Sc, Pt and Si and a, b, c and d are 0 less than a less than 3.0, 0 less than bxe2x89xa62.0, 0xe2x89xa6cxe2x89xa62.0 and 0 less than dxe2x89xa62.0 respectively).
In the above magnetic garnet single crystal according to the present invention, the film thickness is more than 200 xcexcm.
Further, the above objects are achieved by a Faraday rotator which is formed from the above magnetic garnet single crystal according to the present invention. Furthermore, an insertion loss of the Faraday rotator according to the present invention is less than 0.1 dB.
An operation of the present invention is described below. Ti4+, Pt4+ or Ge4+ is mainly substituted to an Fe site of a 6-coordination in a lattice of the Bi-substituted rare earth iron garnet. However, since an ion radius of Ti4+ is larger than that of Fe3+ in the 6-coordination, a distortion is generated in the lattice of the Bi-substituted rare earth iron garnet. Therefore, when the epitaxial growth progresses and the film thickness increases, the distortion in the lattice is accumulated and many crystal defects generate. Since the ion radius of Pt4+ or Ge4+ is smaller than that of Fe3+ in the coordination 6, a distortion is not generated in the lattice of the garnet and even when the thickness of the epitaxial film increases, no crystal defect generates. When the Faraday rotator having a Faraday rotational angle of 45 degrees in light equal to 1.31 xcexcm and 1.55 xcexcm in wavelength is fabricated by using the single crystal which substitutes Fe3+ with this Pt4+ or Ge4+, no defect in the crystal interior is confirmed and no irregularities in the extinction ratio such as being less than 40 dB are generated. A similar effect to such effects of PtO2 and Pt as additives can be also expected when other Pt compound is used. Further, a similar effect to the effect of GeO2 as an additive can be also expected when other Ge compound is used.
Further, the a indicates the amount of Bi in the magnetic garnet in the magnetic garnet single crystal according to the present invention. The Bi amount a is a factor to determine a rotational capacity (deg./xcexcm) of the Faraday rotator and the greater the Bi amount a is, the more the Faraday rotational capacity increases. A preferable Bi amount a of the magnetic garnet single crystal when used as the Faraday rotator is equal to approximately 0.6 to 1.5. When the Bi amount a is less than 0.6, the Faraday rotational capacity is too small and when the Bi amount a is more than 1.5, a deposition of a phase other than garnet occurs, thereby generating a possibility in which normally the magnetic garnet can not epitaxially grow. However, in the present condition, the magnetic garnet which is less than 0.6 in Bi amount a can be experimentally fabricated. Further, the magnetic garnet which Bi amount a is 3.0 can be also obtained by using a vacuum deposition technique. Therefore, the Bi amount a of the magnetic garnet single crystal for fabricating the Faraday rotator is established to be 0 less than a less than 3.0 in the present invention.
The b indicates the amount of Pb in the magnetic garnet. Since the garnet which is at least substantially 2.0 in Pb amount b can exist in the state of a sintered body, 0 less than bxe2x89xa62.0 is established in the present invention.
The c indicates the amount of nonmagnetic elements such as Ca, Al and the like which can be substituted for Fe. If the nonmagnetic element amount c exceeds approximately 2.0, the magnetic garnet turns from a ferrimagnetic body into a paramagnetic body, the Faraday rotational capacity is considerably reduced, thereby making impossible to be used as a rotator. Therefore, the nonmagnetic element amount c is established to be 0 less than cxe2x89xa62.0 in the present invention.
The d indicates the amount of Pt or Ge. Since the amount of Pb which is a divalent element and the amount d of Pt or Ge which is a tetravalent element are required to be substantially the same in order to reduce the light absorption loss, the Pt or Ge amount d is established to be 0 less than dxe2x89xa62.0 similarly to the Pb amount.
Further, 0.5xe2x89xa6b/dxe2x89xa62.0 is desirable for Pt owing to the relation with the light absorption loss (insertion loss). For example, since the Pb amount b is equal to 0.04 and the Pt amount d is also equal to 0.04 in Example 1 described later, (Pb amount b)/(Pt amount d)=1 is realized. The insertion loss of the Faraday rotator is from 0.01 to 0.05 dB at this time. Furthermore, for example, since the Pb amount b is equal to 0.04 and the Pt amount d is equal to 0.02 in Example 2, (Pb amount b)/(Pt amount d)=2 is realized. The insertion loss of the Faraday rotator at this time is from 0.06 to 0.10 dB. The value generally required as the insertion loss of the Faraday rotator for an optical isolator is 0.10 dB. The insertion loss is smallest in a composition in which the Pb amount b is in accordance with the Pt amount d, and as the ratio between the Pb amount b and the Pt amount d are different, the insertion loss increases. Therefore, in order to satisfy the generally required value for the insertion loss of 0.10 dB, a condition of 0.5xe2x89xa6b/dxe2x89xa62.0 is required.
In the embodiment of the present invention, a Bi-substituted rare earth iron garnet single crystal film which contains Pt or Ge and is more than 200 xcexcm in film thickness is grown from flux containing Pb. By using the obtained Bi-substituted rare earth iron garnet single crystal film, a Faraday rotator which is low in light absorption loss and crystal defects and high in extinction ratio can be fabricated with stability.